1. Field of the Invention
The present invention relates to a curable resin for photo-patterning, a process for producing the same, a curable resin composition for photo-patterning, comprising the curable resin for photo-patterning, a color filter using the curable resin composition for photo-patterning, a liquid crystal panel substrate making it possible to maintain an even cell gap, and a liquid crystal panel superior in display quality, using the color filter or the liquid crystal panel substrate.
2. Description of Related Art
In a liquid crystal panel, a display side substrate and a liquid crystal driving side substrate are faced with each other and a liquid crystal compound is put between the two to form a thin liquid crystal layer. The liquid crystalline order in the liquid crystal layer is electrically controlled through the liquid crystal driving side substrate to change the amount of light transmitted through the display side substrate or light reflected thereon, thereby performing display.
For a liquid crystal panel, various driving systems are known, examples of which include a static driving system, a simple matrix system, and an active matrix system. In recent years, color liquid crystal display devices using a liquid crystal panel according to an active matrix system or a simple matrix system have spread rapidly as flat displays for personal computers, portable information terminals and so on.
FIG. 1 illustrates an example of a liquid crystal panel according to an active matrix system. A liquid crystal panel 101 has a structure wherein a color filter 1, which is a display side substrate, is faced with a TFT array substrate 2, which is a liquid crystal driving side substrate, to form a gap part 3 of about 1 to 10 μm; the gap 3 is filled with a liquid crystal L; and the periphery thereof is sealed up with a sealing material 4. The color filter 1 comprises a structure including a black matrix layer 6 formed in a given pattern for shading light from the boundary part between the pixels, a color layer 7 with a plurality of colors (in general, the three primary colors of red (R), green (G), and blue (B)) arranged in a given order for forming each pixel or a recent color layer using a hologram, a protect layer 8, and a transparent electrode layer 9 successively laminated on a transparent substrate 5 in this order from the side close to the transparent substrate. The TFT array substrate 2 has a structure wherein TFT elements are arranged on the transparent substrate and the transparent electrode layer is deposited (not illustrated). Moreover, an alignment layer 10 is provided on the inner surface side of the color filter 1 and the TFT array substrate 2 facing thereto. Furthermore, spherical or rod-like particles 11, which have a constant size and are made of glass, alumina, a plastic or the like, are dispersed as spacers in the gap part 3 for constantly and homogeneously maintaining the cell gap between the color filter 1 and the electrode substrate 2. A color image can be obtained by controlling the light transmittance of the pixels colored in respective colors or the liquid crystal layer behind the color filter.
The protect layer 8 formed in the color filter serves for protecting the color layer and for flattening the color filter when a color layer is provided in the color filter. In a color liquid crystal display device, when the flatness of the transparent electrode layer 9 deteriorates due to the existence of gap unevenness derived from waviness on the surface of the transparent substrate of the color filter, gap unevenness among the R, G and B pixels, or gap unevenness within each pixel, color unevenness or contrast unevenness is generated so as to give rise to a problem of image quality deterioration. Therefore, a high flatness is required for the protect layer.
When the fine particles 11 as shown in FIG. 1 are dispersed as the spacers, the particles 11 are randomly dispersed whether they are behind the black matrix layer 6 or behind the pixels. When the particles 11 are disposed in the display region, that is, in the color layer, a back lighting beam transmits the particle part and further the orientation of the liquid crystal is disturbed in the vicinity of the particles. As a result, the display image grade is remarkably deteriorated. Thus, as shown in FIG. 2, instead of dispersing the particles 11, column-shaped spacers 12 having a height corresponding to the cell gap are formed on the inner surface side of the color filter and in the region coinciding with the black matrix layer 6 (namely, non-display region).
The color layer 7, the protect layer 8 and the column-shaped spacers 12 can be made of a resin. It is necessary that in the color layer 7, a given pattern is formed for pixels in each color. Considering the adhesion property and the sealing property of the sealing part, the protect layer 8 is preferably one capable of covering only the region of the transparent substrate with the color layer formed thereon. Moreover, the column-shaped spacers 12 need to be formed accurately in the region with the black matrix layer formed, that is, in the non-display region. Therefore, it is suggested that the color layer, the protect layer and the column-shaped spacers are formed with the use of a photo-curable resin which can be developed with an alkali after regions to be cured is selectively exposed to light.
As an alkali-soluble photo-curing resin, for example, o-cresol novolak epoxy acrylate having a weight average molecular weight of about 2000 is known. The resin has a carboxylic acid group defining the alkali solubility. However, since the resin uses a monomer component as an acryloyl group defining the curability, the reliability in film formation is low. For example, it involves the risk of the elution of remaining monomer units into the liquid crystal part. Furthermore, the layer thickness may be reduced due to a large amount of the elution in the alkali development.
Moreover, as a method for introducing a radical polymerizable group such as an acryloyl group into the molecular structure of a compound to provide photo curability, there is known, for example, a method of introducing a radical polymerizable group such as a methacryloyl group to an end of the compound by reacting a diol with excessive diisocyanate to prepare a reaction product with an isocyanate group remaining on the end, and reacting the isocyanate group of the reaction product with 2-hydroxyl ethyl methacrylate to produce urethane acrylate. However, according to the method, as a principle, (meth)acryloyl groups can be introduced to only both ends of the molecular structure. Furthermore, the following method is conceivable: a method of conducting radical polymerization in the state that a polyfunctional compound having two or more radical polymerizable groups, such as (meth)acryloyl groups, in the single molecule thereof, is contained. However, the content of the radical polymerizable groups cannot be controlled and further a problem of gellation is also involved.
Thus, the inventors suggested a photo-curable resin having a main chain comprising a constitutional unit represented by the following formula (15) and a constitutional unit represented by the following formula (16) in which a (meth)acryloyloxyalkylisocyanate compound represented by the following formula (17) is bonded with at least one part of carboxyl groups and hydroxyl groups of the main chain by reaction of the isocyanate group of this compound:
wherein R20 represents hydrogen or an alkyl group having 1 to 5 carbon atoms, R21 represents an alkylene group having 2 to 4 carbon atoms, R22 represents an alkylene group, and R23 represents hydrogen or methyl.
A Japanese Patent Application (No. 2000-105456) for this resin was filed.
The photo-curable resin concerned with this suggestion has an advantage that the quantities of the carboxyl groups, which are alkali-soluble, and the (meth) acryloyl groups, which are radical-polymerizable, can be freely adjusted. However, it cannot be said that the resin has sufficient sensitivity. Thus, in order to cure the resin rapidly at a little exposure amount, it is necessary to use a relatively large amount of a photopolymerization initiator. However, if the photopolymerization initiator is used in a large amount, disadvantages as follows are generated: the resin is easily colored; an effect as an impurity becomes large so as to cause particularly liquid crystal pollution; and the photopolymerization initiator bleeds out to the surface of the coating layer so that physical properties of the layer are damaged.
Column-shaped spacers formed using a conventional photo-curable resin used in photolithography are plastically deformed at high temperature and high pressure at the time of assembling a color filter and a TFT array substrate (forming a cell by adhesion under pressure (cell pressure adhesion)). As a result, a cell gap having a given interval cannot be kept even so that display unevenness and so on are generated. Thus, a problem that the spacers are prohibited from fulfilling their original function arises.
The column-shaped spacers made of the curable resin are also plastically deformed by impact or pressing force from the outside after the color filter and the TFT array are assembled to form a liquid crystal panel. As a result, its cell gap becomes uneven to generate display unevenness. Thus, the spacers may also be prohibited from fulfilling their original function.
When the hardness of the column-shaped spacers is made high to prevent such plastic deformation, the spacers cannot follow contraction and expansion of liquid crystal within a wide practical temperature range (−20 to +40° C.), to cause problems that foams are generated in the liquid crystal layer to results in a problem of a drop in display quality, based on color omission (void), color unevenness, and so on.
In order to make the cell gap precise and even by the column-shaped spacers, it is necessary to overcome the above-mentioned problems.
In recent years, an increase in the area of liquid crystal display devices have been advancing, and the necessity that their cell gap should be kept even over the whole of their wide substrate has been becoming larger. When the area of the substrate is large, the substrate is distorted even by a relatively small external force. Therefore, the necessity that the unevenness of the gap based on such a distortion should be prevented has also been generated. In recent years, the thickness of the liquid crystal layer, that is, the cell gap has become smaller in order to improve display responsibility. Thus, the necessity that the small gap should be precisely maintained has also been generated.
Following such a late increase in the display region of liquid crystal display devices and such a late reduction in the cell gap thereof, display performance thereof is largely affected even if the evenness of the cell gap is slightly damaged. Thus, display quality is easily lowered because of display unevenness and so on. Consequently, preciseness and evenness are more and more strictly demanded for the cell gap.
In recent years, in order to remove a heating process and a slow cooling process from the process for assembling a color filter and a TFT array substrate (cell pressure adhesion) so as to make the process simpler and improve productivity, a method of performing cell pressure adhesion at room temperature (room-temperature cell pressure adhesion method) has been suggested.
In order to improve the productivity of the cell pressure adhesion process, a One Drop Fill Technology (ODF technique) is suggested (1354·SID 01 DIGEST, 56. 3: development of One Drop Fill technology for AM-LCDs (H. Kamiya et al.)). In this method, a given amount of a liquid crystal droplet is dropwise added to a liquid crystal enclosing face of a liquid crystal panel substrate such as a color filter or a TFT array substrate, and another liquid crystal panel substrate is opposed and adhered thereto in a vacuum in the state that a given cell gap can be kept. This method can make the processes thereof simpler than any conventional cell pressure adhesion process. According to any conventional cell pressure adhesion process, a color filter and a TFT array substrate are opposed and adhered to each other in the state that a given cell gap can be kept, and subsequently a liquid crystal is filled and put into the cell gap from a filling-opening made in an end of the adhesion product, using a capillary phenomenon and a difference between pressures inside and outside the cell gap. Following the above-mentioned increase in the display region and the reduction in the cell gap, it has been becoming difficult that the liquid crystal is smoothly filled into the gap. On the other hand, according to the ODF technology, a liquid crystal is easily filled into a cell gap even if the area of a liquid crystal panel substrate becomes large and its cell gap is narrow. This new method, which is superior in productivity, may become the main current hereafter.
Japanese Patent Application Laid-Open No. 6-82620 discloses a method of exposing, to light, a layer comprising a photo-crosslinking polymer wherein a monomer (A) having a cyclic imide group, a monomer (B) having an acid group such as (meth)acrylic acid, and a monomer (C) which is different from the monomers (A) and (B) are copolymerized so as to form a protect layer for a color filter. This publication states that the color filter protect layer obtained by using the photo-crosslinking polymer according to the suggestion therein is superior in adhesiveness, heat resistance, and chemical resistance.
Japanese Patent Application Laid-Open No. 2000-235259 describes an active energy ray curable resist which comprises: a copolymer (A) having a constitutional unit made of a compound having an ethylenic unsaturated group and a cyclic imide group, and an ethylenic unsaturated monomer; and a compound having 2 or more (meth) acryloyl groups, the resist being used mainly as a solder resist, an etching resist or a plating resist. This publication states that: the active energy ray curable resist according to the suggestion therein is easily cured by active energy rays; the resist has a superior curability, with no photopolymerization initiator or a small amount of a photopolymerization initiator being added, even when the resist is cured by ultraviolet rays; and the cured product is superior in heat resistance and chemical resistance.